THICKNESS OF PAVEMENT DESIGN BY CALIFORNIA BEARING RATIO .

There are two main types of the pavement constructions, viz, Flexible (bituminous) pavements and Rigid (concrete) pavements. The function of pavement design is to design the thickness and type of the construction that will distribute traffic loads to enable the subgrade (and base or sub base layers) to support these loads. No sound theoretical and fully acceptable method of pavement design has yet been evolved although there are several methods of design in use which are wholly or partly empirical. The thickness design of a pavement involves the interplay of several variables such as the wheel load and its impact effect, volume and character of the traffic expected to use the road, bearing capacity of the soil on which the road is to be built. The individual effect of many of these factors is difficult to evaluate mathematically with any precision. As such when a wheel load is applied to the pavement, the intensity of stress reduces as the depth increases from the top of the surface. Thickness of pavement is increased for heavier loads and a larger total volume of traffic. CBR method of design has been found the most reliable practical means of evaluating the strength of the subgrade (bearing capacity of the soil) and construction materials and of estimating the required thickness of pavement to satisfy a given loading. It is the most convenient and widely used of all the methods. CBR is a measure of the load carrying capacity (resistance to direct penetration) of any soil or granular material, which is expressed as a percentage of the load carrying capacity of a standard crushed rock specimen (which is taken as of 100 percent value) determined by a penetration test. This is an arbitrary figure:a surface having a CBR of 100 percent is one in which a load of 1360 kg has to be exerted to drive in a cylindrical flat plunger with a base area of 19.3 square CM to a distance of 0.250 mm at the rate of penetration of 0.125 mm per minute. It gives reasonably accurate estimate of the required thickness of construction but the CBR method is considered to give extravagant thickness for roads which have to carry only light traffic.(The CBR values are based on laboratory tests according to IS:2720.